Metal complexes of lanthanide metals, especially gadolinium, are of interest as MRI contrast agents in the field of in vivo medical imaging. MRI contrast agents based on metal complexes of gadolinium have been reviewed extensively [see e.g. Zhang et al, Curr. Med. Chem., 12, 751-778 (2005) and Aime et al, Adv. Inorg. Chem., 57, 173-237 (2005)].
Free gadolinium ions do, however, exhibit significant toxicity in vivo. U.S. Pat. No. 5,876,695 addresses this problem by including in the formulation of the gadolinium metal complex an additive, which is a ‘weak metal chelate complex’ such as with calcium. The idea is that the excess ‘weak metal chelate complex’ will complex efficiently any gadolinium ions which may adventitiously be either liberated or present, and thus improve the safety of the MRI contrast composition.
EP 2513043 B1 discloses a method of preparation of gadolinium metal complexes in which gadolinium is first complexed to a cation exchange resin optionally functionalised with a metal coordinating group. The solid-phase bound gadolinium is subsequently reacted with an aminocarboxylic acid chelating agent to liberate the desired gadolinium complex. Any excess gadolinium remains bound to the solid-phase.
EP 2242515 B9 discloses a process for preparing a liquid pharmaceutical formulation containing a complex of macrocyclic chelate with a lanthanide and a mol/mol amount of free macrocyclic chelate of between 0.002% and 0.4%, said process comprising the following successive steps:                b) preparation of a liquid pharmaceutical composition containing the complex of macrocyclic chelate with a lanthanide, and free macrocyclic chelate that is not under the form of an excipient X[X′, L] in which L is the macrocyclic chelate and X and X′ are a metal ion, in particular chosen independently from calcium, sodium, zinc and magnesium, and free lanthanide, by mixing a solution of free macrocyclic chelate and of free lanthanide, so as to obtain complexation of the lanthanide by the macrocyclic chelate, the amounts of free macrocyclic chelate and of free lanthanide being such that not all the lanthanide is complexed;        c) measurement in the pharmaceutical formulation obtained in step b) of the concentration of free lanthanide Clan 1; the concentration of free macrocyclic chelate Cch 1 being equal to 0;        d) adjustment of Cch 1 and of Clan 1 by adding to the formulation obtained in step b) the amount of free macrocyclic chelate necessary, firstly, to complete the complexation of the free lanthanide so as to obtain Clan 1=0, and, secondly, to obtain Cch 1=Ct ch 1, wherein Ct ch 1 is the target concentration of the free macrocyclic chelate in the final liquid pharmaceutical formulation and is selected in the range of between 0.002% and 0.4% mol/mol,wherein the amount of free macrocyclic chelate in the final liquid pharmaceutical formulation corresponds to the proportion of free macrocyclic chelate relative to the amount of complexed macrocyclic chelate in the final liquid pharmaceutical formulation.        
EP 2242515 B9 teaches that the method preferably further includes a prior step a) of determination of the theoretical target concentration of free macrocyclic chelate Ct ch 1 in the final liquid pharmaceutical formulation.
US 2012/0082624 A1 discloses a similar process to EP 2242515 B9, except that the pharmaceutical formulation is obtained in powder form.
Both EP 2242515 B9 and US 2012/0082624 A1 stress that, for an industrial scale pharmaceutical manufacturing processes, the addition of 0.1 mol % free macrocyclic chelator is difficult to achieve with the required degree of accuracy by weighing alone. That was ascribed to the 1000-fold difference in amounts of chelator involved, plus the hygroscopic nature of the chelator. The claimed invention, as described above, is to first carry out the metal complexation with an excess of lanthanide metal ion, then secondly to determine accurately the concentration of uncomplexed, excess lanthanide. That determination is subsequently used to calculate exactly how much additional chelator must be added to both complex the excess lanthanide and achieve the desired 0.1% molar excess of macrocyclic chelate.
Reference Example 3 of EP 2242515 B9 includes a laboratory scale preparation which prepares Gd-DOTA by reaction of DOTA (10 g, 25 mmol) with a stoichiometric amount of gadolinium oxide (Gd2O3, 12.5 mmol) at 80° C. in water at pH 6 to 7. The pH is then adjusted to 5, and residual free gadolinium removed by stirring with a Chelex resin for 2-hours, followed by filtration. EP 2242515 B9 teaches that the Gd-DOTA complex is then precipitated from aqueous ethanol giving an 80% isolated yield of white powder. EP 2242515 B9 does not teach how the method of Reference Example 3 can be adapted to provide the liquid pharmaceutical composition having an excess of macrocyclic chelator in the range 0.002% and 0.4% mol/mol, in particular on an industrial scale.
WO 2014/114664 provides a process for the preparation of Gd-DOTA meglumine (gadoterate meglumine) which first comprises the synthesis of DOTA from cyclen followed by multi-step purification via recrystallisation and both cation and anion exchange chromatography. The purified DOTA is then reacted with Gd2O3 to give the Gd-DOTA complex, followed by the addition of meglumine to give the desired product.
WO 2014/161925 teaches that, when preparing Gd-DOTA and similar complexes on an industrial scale, it is necessary to assay the moisture content of the chelator prior to use and to correct the molar amounts used accordingly. WO 2014/161925 notes that the moisture content of DOTA varies with the relative humidity conditions.
There is still a need for alternative methods of preparing formulations of lanthanide metal complexes of macrocyclic chelators incorporating an excess of such chelators. The methods should preferably be suitable for pharmaceutical manufacture on an industrial scale, and also be suitable for the provision of MRI contrast agents comprising such formulations.